USDA field pilots replaced most woodchips with corn cobs in nitrate bioreactors; the 75% corn-cob beds removed nitrate up to 10 times faster

USDA field pilots have found that when most of the woodchips inside nitrate-removing bioreactors are replaced by corn cobs, farm drainage water is cleaned faster. In a study led by Iowa State University with USDA-ARS scientist Gary Feyereisen, published online in December 2022 in the Journal of Environmental Management, bioreactor beds built with 75 percent corn cobs removed nitrate at 1.6 to 10 times the rate of an all-woodchip bed. Woodchips have been the default material in these systems for over a decade, so a result like this points to a change in how farms across the Midwest could treat drainage water more effectively and, at a lower cost.

What is a nitrate bioreactor, and why should you care

A denitrifying bioreactor is a trench that is buried at the edge of a farm field. It is filled with material and connected to the underground pipes that drain extra water from cropland. As the drainage water moves through the trench, microbes eat the carbon in the buried material. These microbes change dissolved nitrate into nitrogen gas that rises into the air. The US Department of Agriculture has supported this method through its Natural Resources Conservation Service since 2015. This is because much nitrate from farmland is a big reason why waterways in the Corn Belt have pollution problems. It is also why the Gulf of Mexico has a dead zone every summer. Woodchips have been used to fill bioreactors since they began spreading across Iowa, Minnesota, and Illinois. The issue is that woodchips are becoming harder to get in large amounts. They are also more costly to move to fields.


Inside the USDA's corn cob field trial

Researchers did not simply guess that corn cobs might work better; they tested it directly in the field. A team from Iowa State University with USDA Agricultural Research Service scientist Gary Feyereisen built nine field-scale pilot bioreactors. They compared three different carbon mixes: woodchips, a blend of 25 percent corn cobs with 75 percent woodchips, and the opposite blend of 75 percent corn cobs with 25 percent woodchips. Each mix was tested at three water retention times, 2, 8, and 16 hours, the fixed lengths of time water stayed in the bed before draining out, so researchers could see how each material performed under different flow conditions. The findings were published in the peer-reviewed Journal of Environmental Management in December 2022, giving the results the scrutiny needed before any farmer changes farm infrastructure.

The 75 percent corn cob mix came out on top

The bed with 75 percent corn cobs, with a shorter stretch of woodchips near the outlet, converted nitrate to nitrogen gas at a rate 1.6 to 10 times faster than the all-woodchip bed, depending on the retention time tested, a measure of biological efficiency within a given retention window, not of water moving through the system any faster. The bed with 25 percent corn cobs was less steady: it sometimes removed nitrate a little slower than woodchips, and sometimes removed nitrate almost three times faster. Earlier laboratory studies suggested that corn cobs could remove nitrate better than woodchips. This field test proved that the advantage of corn cobs is real when the bed is used outside a laboratory in seasons with changing temperatures.

What this means for farmers and rural water budgets

Corn cobs not only remove nitrate faster, but they also cost less to use. The researchers estimated that 75 percent corn cob bioreactors removed nitrogen for $2.84 to $3.66 per pound versus $3.55 to $27.13 per pound for the woodchip design and $5.95 to $17.99 per pound for the 25 percent corn cob blend. That higher cost range for the 25 percent blend reflects its inconsistent performance described above: cost per pound of nitrogen removed rises sharply whenever a bed removes only a small amount of nitrate relative to its fixed operating costs, which is exactly the scenario the 25 percent mix sometimes produced. Corn is among the widely grown crops in the same Midwest counties where bioreactors are being installed, meaning corn cobs are already present as a byproduct in the exact area that needs the technology most. That local abundance could shorten the distance that materials need to travel and reduce pressure on the woodchip supply chain, which has struggled to keep up with demand.

Final words

This was a field pilot, not a large-scale implementation. The researchers said more long-term studies are needed before we can say for sure how corn cobs hold up over years through different growing seasons and weather conditions. We still don't know how long a bed made from corn cobs lasts compared to woodchips. If more people start using corn cobs, there could be issues with keeping the supply steady and prices reasonable. None of that changes the finding, but it does mean farmers and conservation groups should see this as a hopeful first step, not a final answer.

For Americans who care about food systems and clean water but don't farm themselves, this study shows something important: real environmental progress often comes from places you wouldn't expect. In this case, it came from a farm byproduct that used to be thrown away. If corn cob bioreactors keep working in longer tests, the Midwest might one day clean its water better and cheaper using something already sitting in the field.